Circulating fluid bed (CFB) reactors are well known devices that can be used to carry out a variety of multiphase chemical reactions. In this type of reactor, a fluid (gas or liquid) is passed through a granular solid material at velocities high enough to suspend the solid and cause it to behave as though it were a fluid. Fluidization is maintained by means of fluidizing gas such as air, steam or reactant gas injected through a distributor (grid, spargers or other means) at the base of the reactor. CFB reactors are now used in many industrial applications, among which are catalytic cracking of petroleum heavy oils, olefin polymerization, coal gasification, and water and waste treatment. One major utility is in the field of circulating fluid bed combustors where coal or another high sulfur fuel is burned in the presence of limestone to reduce SOx emissions; emissions of nitrogen oxides is also reduced as a result of the relatively lower temperatures attained in the bed. Another application is in the fluidized bed coking processes known as fluid coking and its variant, Flexicoking™, both of which were developed by Exxon Research and Engineering Company.
Fluidized bed coking is a petroleum refining process in which heavy petroleum feeds, typically the non-distillable residue (resid) from fractionation or heavy oils are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 480 to 590° C., (about 900 to 1100° F.) and in most cases from 500 to 550° C. (about 930 to 1020° F.). Heavy oils which may be processed by the fluid coking process include heavy atmospheric resids, aromatic extracts, asphalts, and bitumens from tar sands, tar pits and pitch lakes of Canada (Athabasca, Alta.), Trinidad, Southern California (La Brea (Los Angeles), McKittrick (Bakersfield, Calif.), Carpinteria (Santa Barbara County, Calif.), Lake Bermudez (Venezuela) and similar deposits such as those found in Texas, Peru, Iran, Russia and Poland. The process is carried out in a unit with a large reactor vessel containing hot coke particles which are maintained in the fluidized condition at the required reaction temperature with steam injected at the bottom of the vessel with the average direction of movement of the coke particles being downwards through the bed. The heavy oil feed is heated to a pumpable temperature, typically in the range of 350 to 400° C. (about 660 to 750° F.) mixed with atomizing steam, and fed through multiple feed nozzles arranged at several successive levels in the reactor. The steam is injected into a stripper section at the bottom of the reactor and passes upwards through the coke particles in the stripper as they descend from the main part of the reactor above. A part of the feed liquid coats the coke particles in the fluidized bed and subsequently decomposes into layers of solid coke and lighter products which evolve as gas or vaporized liquid. Reactor pressure is relatively low in order to favor vaporization of the hydrocarbon vapors, typically in the range of about 120 to 400 kPag (about 17 to 58 psig), and most usually from about 200 to 350 kPag (about 29 to 51 psig). The light hydrocarbon products of the coking (thermal cracking) reactions vaporize, mix with the fluidizing steam and pass upwardly through the fluidized bed into a dilute phase zone above the dense fluidized bed of coke particles. This mixture of vaporized hydrocarbon products formed in the coking reactions continues to flow upwardly through the dilute phase with the steam at superficial velocities of about 1 to 2 meters per second (about 3 to 6 feet per second), entraining some fine solid particles of coke. Most of the entrained solids are separated from the gas phase by centrifugal force in one or more cyclone separators, and are returned to the dense fluidized bed by gravity through the cyclone diplegs. The mixture of steam and hydrocarbon vapor from the reactor is subsequently discharged from the cyclone gas outlets into a scrubber section in a plenum located above the reaction section and separated from it by a partition. It is quenched in the scrubber section by contact with liquid descending over scrubber sheds in a scrubber section. A pumparound loop circulates condensed liquid to an external cooler and back to the top row of scrubber section to provide cooling for the quench and condensation of the heaviest fraction of the liquid product. This heavy fraction is typically recycled to extinction by feeding back to the fluidized bed reaction zone.
Components of the feed that are not immediately vaporized coat the coke particles in the reactor and are subsequently decompose into layers of solid coke and lighter products which evolve as gas or vaporized liquids. During the contacting of the feed with the fluidized bed, some coke particles may become unevenly or too heavily coated with feed and during collision with other coke particles may stick together. These heavier coke particles may not be efficiently fluidized by the steam injected into the bottom of stripper section so that they subsequently pass downwards from the reactor section into the stripper section where they may adhere to and build up on the sheds in the stripper section, mainly on the uppermost rows of sheds. Conventionally, the stripper section has a number of baffles, usually termed “sheds” from their shape in the form of inverted channel sections extending longitudinally in several superimposed rows or tiers across the body of the stripper. The coke passes over these sheds during its downward passage through the stripper and is exposed to the steam which enters from the spargers at the bottom of the vessel below the sheds and is redistributed as it moves up the stripper. The solid coke from the reactor, consisting mainly of carbon with lesser amounts of hydrogen, sulfur, nitrogen, and traces of vanadium, nickel, iron, and other elements derived from the feed, passes through the stripper and out of the reactor vessel to a burner where it is partly burned in a fluidized bed with air to raise its temperature from about 480 to 700° C. (about 900° to 1300° F.), after which the hot coke particles are recirculated to the fluidized bed reaction zone to provide the heat for the coking reactions and to act as nuclei for the coke formation.
The Flexicoking™ process, also developed by Exxon Research and Engineering Company, is, in fact, a fluid coking process that is operated in a unit including a reactor and burner, often referred to as a heater in this variant of the process, as described above but also including a gasifier for gasifying the coke product by reaction with an air/steam mixture to form a low heating value fuel gas. The heater, in this case, is operated with an oxygen depleted environment. The gasifier product gas, containing entrained coke particles, is returned to the heater to provide a portion of the reactor heat requirement. A return stream of coke sent from the gasifier to the heater provides the remainder of the heat requirement. Hot coke gas leaving the heater is used to generate high-pressure steam before being processed for cleanup. The coke product is continuously removed from the reactor. In view of the similarity between the Flexicoking process and the fluid coking process, the term “fluid coking” is used in this specification to refer to and comprehend both fluid coking and Flexicoking except when a differentiation is required.
The stripping section of the fluid coking unit is located in the lower portion of the reactor. Coke particles from the reactor pass into the stripper where they are contacted with stripping steam from a sparger located at the bottom of the stripping section in order to remove hydrocarbon vapor phase products from the coke which is carried out of the bottom of the unit. As a result of the well-mixed nature of the reactor, a certain amount of coke entering the stripper is still coated with crackable hydrocarbon material. For this material, the stripper acts as an additional reaction section within which cracking and drying can occur. As this material progresses through the stripper, additional cracking reactions occur. For this reason, plug flow behavior is extremely desirable in the stripper in order to minimize the amount of crackable material sent to the burner or heater as hydrocarbon carryunder, where it is effectively downgraded to coke. With basic fluid cokers, unlike Flexicokers, this phenomenon is not greatly disadvantageous as the quantities are small but in the case of Flexicokers, this material is sent to the heater, where it is exposed to a high temperature, oxygen poor environment. Unreacted material that enters the heater can crack to form a full range of vapor phase products. These products are then carried up into the heater overhead where they can condense onto surfaces resulting in capacity and/or run length limitations.
While hydrocarbon carryunder is not a major concern for fluid coking units, these units do experience a different type of concern arising from operation of the stripper. Accumulation of deposits on the stripper sheds, which typically take on a characteristic shape by which they are named “shark fins”, makes the stripper vulnerable to reduced clearances that can interrupt the coke circulation in the stripper section, restrict fluidization of the coke in the reactor section and eventually lead to unplanned capacity loss or an unplanned reactor shutdown.
The dense fluid bed behaves generally as a well mixed reactor. However cold flow dynamics model simulations and tracer studies have shown that significant amounts of wetted coke can rapidly bypass the reaction section and contact the stripper sheds where a portion of the wet film is converted to coke, binding the coke particles together. Over time, hydrocarbon species from the vapor phase condense in the interstices between the particles, creating deposits which are very hard and difficult to remove. Current practice in fluid coking units is to raise reactor temperatures to accelerate the thermal cracking reactions. This enables the coke to dry more quickly and thereby reduce the amount of wetted coke that enters the stripper. However the higher reactor temperature increases the rate of recracking of the hydrocarbon vapors and reduces the C4+ liquid yield resulting in an economic debit.
Other attempts have previously been made to overcome this problem with varying degrees of success. One approach has been to improve stripper operation, for example, by fitting the strippers with steam spargers located underneath the stripper sheds, as reported by Hsiaotao Bi et al in “Flooding of Gas-Solids Countercurrent Flow in Fluidized Beds”, In Eng. Chem. Res. 2004, 43, 5611-5619.
Another approach both to reducing reactor fouling and to increase liquid yield has been to improve the atomization of the feed as it enters the bed with the expectation that improved atomization will reduce the extent to which the oil will be carried down in liquid form into the stripper. Conventional atomization nozzles used in the fluid coking process use steam to assist in spraying the heated resid or bitumen into the fluidized bed of hot coke particles: effective contacting of resid droplets and the entrained coke particles is important in improving reactor operability and liquid product yield. The injected spray forms a jet in the bed into which fluidized coke particles are entrained. A major concern in the process is liquid-solid agglomerates tend to form in the bed with poorly performing atomization nozzles, causing high local liquid loading on the solids and the formation of large wet feed/coke agglomerates. These heavier agglomerates may tend segregate towards the lower section of the reactor and foul the internals of the reactor, particularly in the stripper section, as noted above. With enhanced feed atomization performance, the contacting between the spray jets of feed and coke solids would be improved, resulting in an overall improvement in reactor operability, with longer run-lengths due to reduced stripper fouling, and/or higher liquid product yield due to lower reactor temperature operation. Higher liquid feed rates may also be facilitated by the use of improved feed nozzles.
A steam assisted nozzle proposed for use in fluid coking units is described in U.S. Pat. No. 6,003,789 (Base) and CA 2 224 615 (Chan). In this nozzle, which is typically mounted to the side wall of the fluid coker so that it extends through the wall into the fluidized bed of coke particles, a bubbly flow stream of a heavy oil/steam mixture is produced and atomized at the nozzle orifice. The nozzle which is used has a circular flow passageway comprising in sequence: an inlet; a first contraction section of reducing diameter; a diffuser section of expanding diameter; a second contraction section of reducing diameter; and an orifice outlet. The convergent sections accelerate the flow mixture and induce droplet size reduction by elongation and shear stress flow mechanisms. The second contraction section is designed to accelerate the mixture flow more than the first contraction section and as a result, the droplets produced by the first contraction are further reduced in size in the second contraction. The diffuser section allows the mixture to decelerate and slow down before being accelerated for the second time. The objective is to reduce the average mean diameter of the droplets to a relatively fine size, typically in the order of 300 μm as it is reported that the highest probability of collision of heavy oil droplets with heated coke particles occurs when both the droplets and heated particles have similar diameters; thus a droplet size of 200 or 300 μm was considered to be desirable.
The objective behind the nozzle of U.S. Pat. No. 6,003,789 is to produce a spray of fine oil droplets which, according to the conventional view, would result in better contact between the coke particles and the oil droplets. A subsequent approach detailed in concept in “Injection of a Liquid Spray into a Fluidized Bed: Particle-Liquid Mixing and Impact on Fluid Coker Yields”, Ind. Eng. Chem. Res., 43 (18), 5663., House, P. at al, proposes that the initial contact and mixing between the liquid droplets and the hot coke particles should be enhanced, with less regard to the size of the liquid droplets in the spray. A spray nozzle using a draft tube is proposed and a nozzle of this type is also described in U.S. Pat. No. 7,025,874 (Chan). This nozzle device functions by utilizing the momentum of the liquid jet issuing from the nozzle orifice to draw solids into the draft tube mixer and induce intense mixing of the solids and liquid in the mixer and by so doing, enhance the probability of individual droplets and particles coming into contact. As a result, more coke particles were likely to be thinly coated with oil, leading to improvement in liquid yield; the production of agglomerates would be curtailed, leading to a reduction in fouling and the reactor operating temperature could be reduced while still achieving high liquid product yield by reducing the mass transfer limitation on the liquid vaporization process. The actual assembly comprises an atomizing nozzle for producing the jet which extends through the side wall of the reactor and an open-ended draft tube type mixer positioned horizontally within the reactor and aligned with the nozzle so that the atomized jet from the nozzle will move through the tube and entrain a stream of coke particles and fluidizing gas into the tube where mixing of the coke and liquid droplets takes place. The draft tube preferably has a venturi section to promote a low pressure condition within the tube to assist induction of the coke particles and fluidizing gas. This device has not, however, been commercially successful due to concerns over fouling of the assembly in the fluidized bed.
The circular exit orifice on the nozzles shown, for example, in the Base and Chan patents, creates a cylindrical plume of liquid emitting from the nozzle exit; this plume has a minimum area to perimeter ratio and this creates a significant hindrance to the penetration of solid particles to the central core of the jet, possibly leading to contact between the hot coke particles and the injected oil stream which is less than optimal.
An improved mixing arrangement is described in U.S. Pat. No. 7,140,558 (McCracken); this device, referred to here as a Bilateral Flow Conditioner, brings the oil and steam into the main flow conduit leading to the nozzle through feed conduits which are disposed at an acute angle to the main flow conduit and at an angle to each other. In addition, a flow restrictor is disposed in the steam line at the point where it enters the main conduit so that the steam is accelerating as it enters the main conduit. This configuration for the mixing section of the nozzle is stated to give an improvement in flow characteristics.